The field of the invention relates in general to firearms and more particularly, to a firearm for readily accepting a wide variety of interchangeable barrels.
Existing firearm designs which provide for a single action and stock to accept a variety of different barrels suffer from a number of deficiencies and impracticalities which render them generally undesirable to consumers and users. Among such deficiencies and impracticalities are the following.
In certain existing switch-barrel rifles, methods of barrel attachment unduly reduce the dimensions and weaken the barrel in the chamber area, thereby limiting the range of useable cartridges and increasing the likelihood of extraction difficulties and problems in the reloading of fired cartridges.
Some existing switch barrel methods of attachment also do not provide for repeatable installation. Also, conventional methods for providing interchangeable barrels do not fully secure the barrel in the action, thus severely compromising disposed accuracy. For example, U.S. Pat. No. 3,842,527 relies upon a set screw through the receiver ring which engages the barrel threads to prevent rotation. There are significant problems with this approach.
In order to assure proper machinability and smooth interior finish, virtually all commercially available barrels are made from relatively unhardened steel, with Rockwell "C" hardness ranges in the high 20's and low 30's. In contrast, set screws, because of the very nature of the work they do, are always hardened to a much greater degree. Thus, a set screw such as the one used in U.S. Pat. No. 3,842,527 must be screwed in against the soft barrel threads and will deform those threads quite easily, leading to difficulty in removing the barrel and other operational problems.
Another disadvantage of conventional methods for securing a barrel to an action in switch-barrel rifles is that there must be some clearance between the threads of the female action and the male barrel threads, or it will be impossible to screw the action and barrel together. A conventional side engaging set screw will always thrust the installed barrel to the other side, opposite the set screw to some degree. This results in a barrel which will not always be in uniform coaxial alignment with the action. Virtually all barrel threads are less than one inch in length, and many are three-quarter inch or less. A side deflection of only 0.001 inches in a span of 0.750 inches will produce a point of impact deflection of 4.8 inches at 100 yards. Such a conventional method for attaching a barrel produces an utterly unsatisfactory situation from an accuracy standpoint.
Other existing methods of barrel attachment may unduly reduce the size and weaken the barrel in the chamber area, thereby limiting the range of useable cartridges and increasing the likelihood of extraction difficulties and problems in the reloading of fired cartridges. Such conventional methods of barrel attachment are also conducive to damage and deformation of the barrel. For example, many conventional sporting rifles have a muzzle diameter of no more than 0.550 inches. This means that male hexagonal wrenches used to disengage such barrels under this scheme are, for practical purposes, limited to about 5/16th inch across the flats. Machine screws using this tightening method are universally highly hardened, while the steel of a rifle barrel is comparatively quite soft and easily deformed. Over time, this method of attachment results in a barrel that becomes battered and disfigured in the area of the wrench engagement.
Other known methods of barrel attachment do not provide for repeatable installation and do not fully secure the barrels in the action, thus compromising accuracy. Such barrel attachment in certain cases has resulted in heavy or bulky components which compromise accuracy due to the inability of the barrel to be truly free floating and to vibrate at a natural resonant frequency when fired. The most accurate small arms in the world (known as "bench rest rifles") virtually all have free floating barrels. That is, there is no contact between the forearm and the barrel. A free floating barrel insures that barrel harmonics remain as uniform as possible from shot to shot and thereby eliminate any tendency of the forearm to transfer a variable external load to the barrel. Accordingly, a free floating barrel allows the barrel to vibrate at its natural resonant frequency. When the barrel is clamped or otherwise engaged so that its natural resonant frequency may be altered shot-to-shot, repeatable accuracy becomes impossible to achieve.
U.S. Pat. No. 1,517,328 addresses the foregoing problem by providing a design comprising a barrel within a barrel. The replacement barrel is secured to the primary barrel at the muzzle. However, there are both safety related and practical reasons why this design is not adaptable to modem large caliber, high pressure rifle loads. Modem high velocity rifle cartridges normally operate at pressures of up to 60,000 pounds per square inch and frequently generate muzzle energies of 3,000 foot pounds or more. For this reason, barrels must be very strong over the chamber section, and must be rigidly locked to the action/bolt assembly during firing. A barrel diameter over the chamber of at least 0.950 inches is generally considered minimal for such high velocity cartridges. Even this configuration has yielded ruinous barrel deformation with certain high pressure magnum loads. Since the replacement barrel under the attachment scheme of U.S. Pat. No. 1,517,328 must be inserted from the muzzle, the outside barrel or sleeve must be well over one inch in diameter for its entire length. This leads to an unaesthetic, bulky and grossly heavy firearm, totally unsatisfactory for many applications.
The conventional switch barrel apparatus disclosed in U.S. Pat. No. 1,517,328 also requires clearance between the inner and outer barrels. The stresses generated upon firing will cause the inner barrel to vibrate at its own resonant frequency and to be displaced in an unpredictable manner within the outer barrel. This further destroys reliable accuracy.
Even high quality steel is somewhat elastic. For this reason, firearm designers working with high intensity cartridges will condense the locking together of the bolt, action, and barrel in order to avoid excessive stretching during firing. Excessive stretching leads to a wide range of operational difficulties, including hard extraction, short cartridge case life and poor head space control. Most center fire rifles designed for high pressure cartridges accomplish the locking function within a span of two inches or less. However, the design of a switch barrel rifle applying the conventional technique of U.S. Pat. No. 1,517,328 would yield a locking span of approximately two feet, a totally undesirable and impractical configuration.
A further conventional method of providing a firearm with multiple interchangeable barrels is shown in U.S. Pat. No. 4,288,938. Barrel installation is accomplished through conventional threads on the firearm's frame and barrel, modified only slightly and secured by two set screws mounted perpendicular to the barrel's axis. However, recognizing that there must be clearance between the barrel threads and frame threads, such set screws when tightened act to thrust the barrel laterally within the frame, creating inconsistencies in a bullet's point of impact. The fact that U.S. Pat. No. 4,288,938 relies upon two set screws, each acting independently and in opposition to the other, means that there will be a large positional variation as to potential lock-ups, thus creating an unsatisfactory arrangement when accuracy is important. In addition, the arrangement shown in U.S. Pat. No. 4,288,938 deals exclusively with revolving cylinder type firearms.
U.S. Pat. No. 4,674,217 provides a reversible barrel concept wherein an underlug is machined or otherwise installed on both ends of the barrel to engage with a locating pin. This requires a separate manufacturing step which adds significantly to the complexity and thus the cost of producing a rifle barrel.
Also, since there must be a firing chamber at each end of the barrel, the barrel must have a heavy chamber section at each end. This disadvantageously produces an ungainly, bizarre appearing barrel profile and generates unacceptably high weight for certain applications. There is no feasible means for providing a reliable barrel crown. There is broad agreement among experts in firearm fabrication that, in order to provide a reliable crown, the profile of the crown must depart the line of the bore by an angle of at least 30.degree. and up to 90.degree.. The design of U.S. Pat. No. 4,674,217 necessarily uses a firing chamber throat as a crown. Virtually all throats are machined to an angle of 1.5.degree. to 3.5.degree. from the line of the bore, far too shallow to provide an accurate crown.
U.S. Pat. No. 3,138,889 provides another barrel within a barrel concept that is adaptable only to larger gauge shotguns. Very few shotguns currently in production are designed to withstand the pressures generated by high velocity rifle cartridges, thus effectively limiting the potential use of this conventional design to shot shells and low powered metallic ammunition. The conventional design of U.S. Pat. No. 3,138,889 is also only adapted to break-open firearms, which are not desirable from an accuracy standpoint.
U.S. Pat. No. 3,731,418 is another conventional system permitting barrel switching. It relies on a longitudinally slit receiver ring with a lateral clamping mechanism. As a consequence it requires its own unique receiver design which is not adaptable to other rifles.
All but one of the embodiments disclosed in U.S. Pat. No. 3,731,418 require that the action be removed from the stock in order to effectuate a barrel change. This is disadvantageous in that the process of removing the action becomes time consuming and offers greater potential for parts loss. A further embodiment relies on a lever within a separate forearm to provide the barrel clamping function, acting upon helical cams laterally mounted within the receiver ring. This adds weight, complexity and cost to the system. Moreover, a modest pull on the forearm would disengage the barrel clamp, producing a dangerous condition and destroying accuracy if the rifle is being fired at that moment.
U.S. Pat. No. 3,877,167 shows another conventional device for mounting barrels to actions and makes as a secondary claim the ability to quickly change barrels. However, this conventional technique requires a separate barrel extension be machined for each barrel. Such barrel extensions must be carefully machined and hardened and then correctly installed on their respective barrels, adding substantially to costs. This design also requires that the barrel reside within the barrel extension, which must in turn reside within the receiver. Thus, this design requires that the barrel must be smaller than a typical diameter, or that the receiver must be larger than is typical.
The conventional interchangeable barrel apparatus shown in U.S. Pat. No. 3,877,167 further requires a slip fit between the barrel extension and the receiver. Similarly, a slip fit also must exist between the threaded bushing and the barrel. Thus, the barrel may be secured in any number of slightly different positions within the action. As earlier noted, a shift of only 0.001 inch in the short span of a receiver ring can produce a point of impact of over 2 inches at 100 yards. Such a result is inconsistent with repeatable accuracy.
A further conventional method for interchanging barrels is the so-called Gamman Switch Barrel system. The Gamman system requires either a threaded adapter, which threads into the action to be used in lieu of the barrel, or a specially machined action. The barrel is then specifically machined to fit within this adapter. Both adapter and barrel require that large stop-collars be incorporated into each; these collars bear against each other where the barrel is properly attached to the action. A spring-loaded plunger mounted on the adapter parallel to the axis of the barrel engages a notch on the barrel's stop-collar. A threaded dust cover is then secured over the joined stop-collar. The Gamman scheme suffers from undue complexity and is unavoidably both bulky and heavy since it relies on the addition of a significant volume of metal to the existing barrel and action.
The additional weight added to the barrel and action makes the Gamman system disadvantageous for both lightweight sporting rifles and weight limited target rifles. The system is also fundamentally aesthetically unappealing. For example, a large lump of metal is added forward of the receiver ring, which destroys the normally sleek lines of a fine sporting rifle. A large notch must be cut out of the forearm to accommodate the stop-collar assembly, thereby yielding an unwelcome interruption of the stock's profile.
The aforementioned stock cut weakens the stock significantly in both the vertical and lateral planes. This increases both the risk of stock breakage and a tendency of the forearm to deflect under pressure. Another disadvantage of this system is its apparent inability to provide an adapter which reasonably fits within the action. Because the barrel threads must fit within the adapter, which must itself fit within the action, the barrel threads must be reduced in diameter to an unreasonably small size (approximately 0.750 inches). In contrast, virtually all centerfire rifles designed for cartridge heads of 0.473 and above have barrel threads of at least 0.980 inches in diameter, thereby providing a much greater margin of safety and resistance to unwelcome expansion when being fired.
Further, the Gamman system requires a comparatively large amount of high precision machining in order to achieve its operational objectives. This makes it very expensive in relative terms. For example, the price of a barrel conversion is presently quoted at approximately $1,365. For this sum it would be possible to purchase a good quality factory target rifle with optical sights as well as precision gunsmithing work. Therefore, this method is not cost effective.
Another conventional method for attaching different barrels to an action is the so-called Savage barrel nut method employed by Savage Manufacturing Company. This involves a barrel set-nut having the same interior threads as the receiver, which threads are used to lock the barrel and action together in the correct relationship. However, since this method uses the comparatively coarse barrel threads, it lacks the same degree of mechanical advantage possessed by the subject invention, and in practice, requires a high degree of torque in locking together the barrel and action. This produces an unwelcome degree of difficulty in disassembling the barrel and action, thereby rendering this method unsuitable for switch-barrel use. Additionally, such disassembly cannot take place with the action in the stock, due to the design of the locking barrel set nut. This fact makes the spanner/nut engagement subject to slippage during a barrel removal operation, and makes this method unsuitable for repeated barrel switching. The Savage, Inc. barrel nut method is merely an inexpensive way of quickly setting headspace on mass-produced actions and barrels. Thus, such an engagement system cannot accommodate the expedient switching of barrels of different calibers.
Another system for attaching a barrel to an action is embodied in U.S. Pat. No. 5,020,260. This is a system providing for a "take-down" rifle (one which may be readily broken in two and conveniently packed). However, it relies on an interrupted thread design, which disadvantageously weakens the action-to-barrel joint. Additionally, the barrel must be firmly attached to the stock fore-end, eliminating the possibility of a free-floating barrel. U.S. Pat. No. 5,020,260 has a tensioning nut, but it does not have the benefit of separate fine-pitch threads, relying instead on an extension of the standard barrel threads. Therefore, such a system lacks mechanical advantage. In addition, the tensioning nut bears against a barrel bracket rather than the action itself, which design limits the adjustment to less than 0.001 inch. U.S. Pat. No. 5,020,260 teaches a complex and comparatively costly system; if a user wished to have additional calibers for the same action, each barrel/forearm assembly is presently priced at $1,000.00. Thus, such a system requires either an expensive proprietary action, or extensive and costly modifications to a very limited range of commercially available actions.
In order to overcome the disadvantages and shortcomings of conventional switch barrel rifle systems, what is needed is a method and apparatus for securely locking a barrel to an action without deforming the barrel in any fashion and thus preserving barrel accuracy. Such a method and apparatus ideally would provide an invariant coaxial alignment of the action and barrel. That is, upon the removal and reinstallment of a given barrel, the rifle must automatically return to its target zero position.
What is needed is a convenient and speedy method and apparatus for barrel disassembly and interchangeability with barrels of different calibers without compromising weight as well as accuracy of the recipient firearm, and without the requirement for heavy or bulky tools, or complex and time-consuming procedures.
It also would be desirable to provide a switch barrel rifle capable of interchanging a plurality of barrels such that the barrels are kept in a completely free-floating mode in a manner similar to the majority of precision bench rest rifles. This advantageously would preserve accuracy of the highest order while permitting interchangeable barrels of different calibers to be used in the same firearm.
What is also needed is a fast and reliable method and apparatus for enabling interchangeable barrels to be employed with a wide variety of currently available actions and not require a complex, proprietary action.
What is also needed is a method and apparatus for securely holding the interchangeable barrels in the action, yet without deformation, such that accuracy is maintained.
It also would be desirable to provide a method and apparatus enabling the ready exchangeability of barrels without removing the action from the stock, (a necessarily time-consuming procedure which can increase the potential for parts loss).
What is also needed is an interchangeable barrel arrangement which retains the typical architecture of directly attaching the barrel to the action. This advantageously would aid in strength, compactness and lightness.
A method and apparatus enabling the ready interchangeability of barrels should accommodate free-floating barrels having a wide variety of possible weights and diameters. It also would be advantageous if the system could be readily adaptable to a variety of currently produced actions without requiring significant modification to those actions. It also would be desirable to provide a solid, repeatable lock-up of the barrel to the action such that the means for joining the barrel to the action would not come loose under heavy usage.